The present invention relates to a bearing for use with a spindle motor, and a spindle motor having the bearing. The spindle motor is used for a memory apparatus such as hard disc drive and a polygon mirror drive apparatus for a bar code reader.
A bearing of the spindle motor for use with, for example, the memory apparatus and the polygon mirror drive apparatus needs a rotational stability, wear-resistance, and vibration-resistance. In particular, a relative inclination and thereby a contact between a shaft and a sleeve in the bearing result in an unstable rotation, which is seriously problematic and then (causes functional damages to the memory and drive apparatuses. A variety of techniques have been developed to avoid such problems.
Among other things, the Japanese Patent Publications Nos. 11/18357 (A) and 11/55918 (A) disclose a technique in which a stator and a rotor magnet are positioned eccentrically in order to force the shaft or sleeve of the bearing in a certain direction, thereby forming a stable rotation thereof.
FIG. 7 shows an example of disclosed in the Japanese Patent Publication JP 11/55918 (A). In the drawing, the stator has a plurality of blades 1 extending radially. Provided around the stator is a rotor 2. The rotor 2 supports a rotor magnet 3 on an inner periphery thereof. A certain gap is defined between the rotor magnet 2 and the blades 1. Each blade 1 has a coil (not shown) wound therearound. Therefore, an application of a voltage to the coils causes a relative rotation between the blades 1 and the rotor magnet 3.
In the drawing, one blade 1a of the stator is designed so that it has a shorter radial length than those of remaining blades 1. This forms a gap h1 between the blade 1a and the rotor magnet 3, which is greater than gap is h2 between the remaining blades 1 and the rotor magnet 3. Due to this, an attraction force (or repellent force) between the blade 1a and the rotor magnet 3 is reduced at this portion, which forces the rotor 2 in that direction.
In the motor disclosed in Japanese Patent Publication No. 11/55918 (A), in order to force the shaft toward the sleeve in a parallel fashion, a position where the stator and the rotor magnet oppose to each other should be positioned at about the center of the bearing with respect to an axial direction thereof. This arrangement is hardly realized due to design restrictions of the bearing.
FIG. 8 illustrates a spindle motor disclosed in the Japanese Utility Model Publication No. 55/36456 (A). The spindle motor has a bearing with a sleeve 4 and a shaft 5 that rotates about the sleeve. The shaft 5 securely holds a rotor 6 to which a magnetic member 7 in the form of ring is mounted. A housing that securely holds the sleeve 4 is provided with a magnet 8 at a portion thereof facing the magnetic member. The magnet 8 attracts or repels the magnetic member, causing the rotor 6 and the shaft 5 to be inclined against the sleeve 4 in one direction during rotation.
The bearing of the spindle motor disclosed in the Japanese Utility Model Publication No. 55/36456 (A) results in that only the distal end of the shaft 5 supported by the sleeve 4 contacts with the sleeve. This wears the specific portions of the sleeve 4 and the shaft 5, which reduces a durability of the bearing.
FIG. 9 illustrates another bearing disclosed in the Japanese Patent Publication No. 11/37157. In this bearing, a shaft 9 is inserted in a sleeve 10 to form the bearing. An inner peripheral surface of the sleeve 10 is formed with tapered sections so that at each tapered section an inner diameter of the sleeve is enlarged from the center of the sleeve toward the its end with respect to the axial direction.
The bearing disclosed in the JP No. 11/37157 (A) aims to prevent a possible leakage of lubricant in the lubricated bearing. Another tearing (not shown) is provided for the shaft 9, causing the shaft 9 to be supported at its opposite ends by the bearing shown and another bearing not shown. This means that the bearing disclosed does not correspond to a bearing according to the present invention in which a shaft or a sleeve is rotated while one is inclined relative to each other with respect to an axis of the bearing.
In sum, conventionally various bearings have been disclosed in which the shaft or sleeve is rotated while either of which is inclined relative to each other in one direction in order to stabilize the rotation of the spindle motor. Each of the hearings, however, has a problem that the shaft makes a contact at its end and thereby the durability of the bearing is reduced. The wearing of the shaft and sleeve can not be solved effectively even with the existence of chamfered portions formed at the opposite ends of the shaft. Therefore, another technique has been desired to solve those problems.
An object of the present invention is to provide a bearing that eliminates disadvantages in the conventional techniques, attains a stable rotation, hardly be influenced by an external swing, and has an improved durability. Further, another object of the present invention is to provide a simple structure and reliable spindle motor.
To overcome the problem, the present invention either shaft or sleeve is rotated relative to the other while it is inclined to the other, causing a higher hydrodynamic pressure at certain portions of the bearing and, by the use of the hydrodynamic pressure, to realize a stable rotation.
Specifically, in a first embodiment of the present invention a bearing for a spindle motor, includes a shaft and a sleeve in which the shaft is inserted. The sleeve has three bearing sections, a section A extending a distance of L1 from one end thereof in an axial direction, a section B extending a distance of L2 from the opposite end thereof in the axial direction of the bearing, and a section C extending a distance of L3 between sections A and B. Each of sections A and B has a tapered surface portion formed therein in which an inner diameter thereof is increased from inside to outside of the bearing. The section C has a cylindrical surface portion formed therein and extending substantially parallel to the axial direction. An inclination of the tapered surface portion of the sections A and B is defined by xe2x80x9cf/L3xe2x80x9d, wherein xe2x80x9cfxe2x80x9d is determined as a gap between an inner surface of section C of the sleeve and an outer surface of the shaft in section C in a condition where the shaft is inclined relative to the sleeve.
In another embodiment of the present invention, a bearing for a spindle motor includes a shaft and a sleeve in which the shaft is inserted. An outer surface portion of the shaft that opposes an inner surface portion of the sleeve and thereby defines a bearing portion of the bearing is formed by three sequential sections, a section (a) extending a distance of L1 from a point that opposes one end of the sleeve in an axial direction thereof, a section (b) extending a distance of L2 from a point that opposes the opposite end of the sleeve in the axial direction, and a section (c) extending a distance of L3 and connecting sections (a) and (b). The intermediate section (c) has a cylindrical surface portion formed therein and extending substantially parallel to an axial direction thereof. Each of sections (a) and (b) has a tapered surface portion formed therein in which an outer diameter thereof is decreased continuously in a direction away from section (c). An inclination of the tapered surface portion of sections (a) and (b) is defined by xe2x80x9cf/L3xe2x80x9d, wherein xe2x80x9cfxe2x80x9d is determined as a gap between an outer surface of the section (c) of the shaft and an inner surface of the sleeve in section (c) with respect to an axial direction of the axial direction of the sleeve, in a condition where the shaft is inclined relative to the sleeve.
Another aspect of the embodiment according to the present invention is directed to a hydrodynamic bearing in which either of the shaft or sleeve rotates while it is biased by an external force and then inclined toward a certain direction, relative to the other with an inclination of xe2x80x9cf/L3xe2x80x9d.
Another aspect of the embodiment according to the present invention is directed to a hydrodynamic bearing in which the inclination of the shaft relative to the sleeve is equal to or less than 0.05 degrees.
Another aspect of the embodiment according to the is present invention is directed to bearing in which, when assumed that the bearing portion has a length of L which is the sum of L1, L2, and L3, the lengths L1 and L2 satisfy following relations:
L1/Lxe2x89xa7xc2xc
L2/Lxe2x89xa7xc2xc
Lxe2x89xa7L1+L2xe2x89xa7L/2
Another aspect of the embodiment according to the present invention is directed to a bearing in which the biasing force is generated by a ring-like magnet eccentrically arranged on an upper surface of the sleeve and another ring-like magnet mounted around the shaft so that the magnets repel to each other.
Another aspect of the embodiment according to the present invention is directed to a bearing in which either or both of the shaft and the sleeve are made of ceramic.
Another aspect of the embodiment according to the present invention is directed to a spindle motor with any one of the above described hydrodynamic bearings.
Another aspect of the embodiment according to the present invention is directed to a spindle motor in which the stator for driving the spindle motor includes a plurality of blades, one of the blades being designed so that, with respect to a rotational axis, an upper end thereof is longer than a lower end thereof, another of the blade positioned on opposite side with respect to the rotational axis being designed so that a lower end thereof is longer than an upper end thereof.
With the embodiments of the present invention, the shaft and sleeve close to each other at the opposite sides with respect to the axial direction and thereby generate a higher hydrodynamic pressure at the close points, realizing a stable rotation without any shaft vibrations.
In the bearing of the present invention, portions of the sleeve and shaft that generate higher hydrodynamic pressure have a sufficient length and therefore, by the use of the bearing of the present invention, a spindle motor with a vibration resistance, long lifetime, and high reliability can be obtained.
Also, by the use of the biasing means for inclining the shaft relative to the sleeve, a stable biasing force is attained. With this means, a spindle motor with a simple structure and a high reliability can be obtained.
Further, the shaft and/or sleeve of the bearing is made of ceramic, which allows to provide a bearing and a spindle motor with the bearing, both having an increased durability and a small size.